1. Field of the Invention
The present invention relates to an imaging apparatus in which a curved image plane (i.e., curved image surface) is disposed near an image plane of an imaging optical system. In particular, the present invention provides an imaging apparatus suitable for a digital still camera, a digital video camera, a cell phone camera, a monitoring camera, and the like.
2. Description of the Related Art
There are disclosed some examples of the imaging apparatus using such a curved image plane. There is proposed a ball lens in which a spherical shell lens and a spherical lens disposed inside the lens constitute concentric spheres (Japanese Patent Application Laid-Open No. S63-081413).
This ball lens can appropriately correct spherical aberration and chromatic aberration.
In addition, because of the point symmetry structure of the ball lens, it is easy to realize a wider angle of field, and hence the ball lens is suitable for an imaging optical system of an imaging apparatus required to have a wide angle and high resolution.
There is disclosed an example of the ball lens in which an aperture stop is disposed on a plane passing through the spherical center of the ball lens in order to obtain good imaging performance by blocking harmful light such as flare.
In addition, there is proposed an imaging optical system in which an image sensor surface is curved in a two-dimensional manner (Japanese Patent No. 4628781).
Japanese Patent No. 4628781 discloses an example in which, in order to reduce constraint in a case where an image sensor serves as an image recording medium, a light receiving plane of the image sensor is made concave to the object side so that an incident angle of a light beam to the image sensor becomes closer to the normal angle.
There is disclosed an optical system formed of only two single-lens members and an aperture stop therebetween, in which one of the lens members has aspherical surfaces on both sides for the purpose of use in an inexpensive camera having a curved image plane (Japanese Patent Application Laid-Open No. H08-338944). This optical system has a full angle of field of at least 62.5 degrees and is an imaging apparatus for a disposable camera in which an image is formed on a curved film plane.
In an imaging apparatus in recent years, a pixel size of the image sensor has been rapidly reduced, and the imaging optical system is required to have a higher resolution. Therefore, the imaging optical system is required to realize high imaging performance even at a small F value.
Further, in recent years, the imaging apparatus are required to have a wider angle and a smaller size.
Japanese Patent Application Laid-Open No. S63-081413 discloses an example of using a ball lens, and, in a main example, an imaging optical system has F/2.8 and has good imaging performance in which spherical aberration and axial chromatic aberration are appropriately corrected.
On the other hand, there is also disclosed an example in which an F value is smaller than F/2.8. For instance, there is disclosed an imaging optical system having F/2.0 or F/1.4. However, in such a small F value, large spherical aberration is generated, and sufficient imaging performance cannot be obtained.
In other words, in the imaging optical system having an F value smaller than F/2.0, in particular, spherical aberration cannot be corrected sufficiently only by the ball lens, and hence there is a problem in that the imaging performance is deteriorated. In addition, there is also disclosed an imaging optical system having F/1.0, in which aberration is corrected by using a high refractive index material such as N=2.500 or N=2.301.
Such a high refractive index material is expensive, and there may cause a problem in that transmittance thereof is deteriorated.
Japanese Patent No. 4628781 discloses an example using a point asymmetry imaging optical system, in which aberrations such as spherical aberration, axial chromatic aberration, and field curvature are corrected in a relatively small F value of F/2.45 to F/2.91 of the imaging optical system, and hence high imaging performance is realized. However, in the imaging apparatus in recent years, a pixel size of the image sensor is reduced to be very small, and an optical system having a higher resolution is demanded. Therefore, in the imaging optical system having an F value smaller than F/2.0, it is a problem in that aberration such as spherical aberration occurs, resulting in deterioration of the imaging performance.
In the optical system of Japanese Patent Application Laid-Open No. H08-338944, the image plane is curved, and the two single-lens members have aspherical surfaces. Thus, the imaging performance is improved over the wide angle. However, this optical system reduces generation amount of spherical aberration by narrowing focus to F/8.0. In addition, in this optical system, the surface closest to the object side is provided with an aspherical surface that is displaced from the reference spherical surface toward the image side, and the surface closest to the image side is provided with an aspherical surface that is displaced from the reference spherical surface toward the object side. In other words, these aspherical surfaces are displaced from the reference spherical surfaces to the inside of the imaging optical system in peripheries of the lens surfaces disposed on the outermost sides of the imaging optical system. The aspherical surfaces correct coma and astigmatism. Therefore, spherical aberration cannot be corrected appropriately.